The oxyhaemoglobin dissociation curve (see Figure 101) describes oxygen binding to haemoglobin and its release in terms of haemoglobin saturation as a function of partial pressure oxygen. It is a sigmoid curve and it rather resembles the drug dose–effect relationship curve (see the chapter on pharmacology). However, in the case of the oxyhaemoglobin dissociation curve, the ‘dose’, i.e. oxygen partial pressure, is on a linear scale. This means that small, linear increments in oxygen partial pressure produce large changes in haemoglobin saturation on the straight part of the curve; at the extreme ends of the curve the opposite applies.

The central value on the straight part of the curve, which corresponds to haemoglobin saturation of 50%, is called the p50; it is normally 3.6 kPa (26.6 mmHg). This is an unphysiological but convenient value to describe the position of the oxyhaemoglobin dissociation curve with respect to the pO2. Points on the normal curve that correspond to the arterial and venous point are denoted as A and V. A shift of the oxyhaemoglobin dissociation curve to the right results in a higher p50, and conversely a left shift lowers the p50. The higher the p50, the lower the affinity of haemoglobin for O2.

Oxygen affinity (and therefore P50 in a reciprocal manner) is affected by the following interrelated factors:

• 2,3-Diphosphoglycerate (2,3-DPG) concentration in the red cell: this is the primary control mechanism. 2,3-DPG is the allosteric effector that binds selectively to the deoxygenated haemoglobin, producing changes in haemoglobin molecule conformation. Oxygen affinity of haemoglobin is inversely related to 2,3-DPG concentration, therefore the oxyhaemo-globin dissociation curve is shifted to the right with a rise in 2,3-DPG.

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